(1) Field of the Invention
Subject invention is related to signal processing in general and more particularly to a multi-stage system using adaptive filters for canceling noise without affecting the signal and thereby increasing signal-to-noise ratio, i.e., S/N.
(2) Description of the Prior Art
In my U.S. Pat. No. 4,589,137 and my pending patent application, Ser. No. 220,692, filed July 5, 1988 which are both incorporated here in entirety, one-stage noise-reducing systems were discussed wherein a single tone's S/N ratio was increased 17 dB by causing the noise in nearby frequency bands to attenuated by 17 dB. Extension of this work resulted in the "unmasking" of four tones (masked by broadband noise) at normalized frequencies of, roughly: 1, 2, 3 and 5 spread over a decade. The original masking noise was reduced over more than a decade of frequency by anywhere from 15 dB to 25 dB.
The adaptive filter using the Least Mean Squares (LMS) algorithm favors noise peak region's and tends not to favor noise dip-regions, so that the 15 dB attenuation occurred at dip regions while the 25 dB attenuation occurred at peak regions. A reasonable number would be 20 dB for the average attenuation of noise attained across a broadband. However, 20 dB attenuation is not sufficient for many applications, and so an attempt to cascade two or more stages of adaptive filters seemed worthwhile in order to try to attain 35 dB or 40 dB of attenuation.